Prior to the 1950s the most successful American 6mm/.243-inch cartridge was the 6mm Lee Navy. But when the .308 Win. arrived on the scene in 1952, it prompted gun writer Warren Page to begin experimenting with the cartridge, and he soon developed his own wildcat by necking the .308 down to .243 (6mm).
Winchester liked what this wildcat could accomplish and legitimized it in 1955. That same year Remington launched its own 6mm cartridge, known as the .244 Rem., which was based on the .257 Roberts. The Remington round had a slight ballistic edge over the .243 Win., and a brief war for 6mm dominance ensued, but the .243 prevailed.
Few challengers have taken on the .243 since then: the speedy .240 Wby. and the ill-fated .243 WSSM—as well as competition-centric rounds like the 6XC and the 6x47 Lapua. But the .243 Win. has largely enjoyed being the king of 6mm mountain.
Hornady's launch of the 6mm Creedmoor as a commercial cartridge (the initial wildcat was the brainchild of Outdoor Life's John Snow) coincided with the rise of a new rifle discipline, too—Precision Rifle Series competition or PRS. 6mm cartridges were common for this because, with high BC bullets, you got good trajectory and mild recoil. Further it could be loaded with full-length bullets and still function in an AR-10, so there were more options for competitive rifle platforms.
If you're a deer hunter who dearly loves his .243 Win. your eyes have already glazed. You probably don't care about the nuances of PRS shooting, and you're wondering why the world is so excited about another new 6mm cartridge. Fair enough.
The .243 Win. has several significant advantages over the 6mm Creedmoor that are primarily the result of the .243's six-decade head start in development. First, virtually all bolt-action hunting rifles are available in .243, and there are also AR-10 variants as well (though they won't handle really long, high BC bullets if that matters to you.)
Plus, there are a whole bunch of different ammo options available to you, everything from 55- or 58-grain varmint loads to 100-grain big game loads, and if you want to shoot competition there are heavier bullets for the .243 that will work. Load data abound.
While the 6mm Creedmoor is very new, there is already great interest in it. Most competition shooters now shoot 6mms, and the 6mm Creedmoor is the most popular option. That's quite impressive for a new cartridge.
There aren't as many rifle options in 6mm Creedmoor as there are in .243, to be sure, but there are a number of 6mm Creedmoor rifles available from a host of major manufacturers: Browning X-Bolt Hell's Canyon Long Range, Savage 10T-SR, Montana Rifle Company's MSR, ASR and X2 and Ruger's Precision Rifle and American Rifle Predator, to name but a few.
Hornady is currently offering its 108-grain ELD Match load at 2,960 fps as well a brand-new 103-grain Precision Hunter load. 6mm Creedmoor brass is more difficult to find than .243 brass, but Hornady is now producing brass for the cartridge. The 6mm Creedmoor's case has a 30-degree shoulder that makes it slightly more efficient than the .243 and promotes case longevity, which benefits for reloaders.
Who's the winner? That depends. If you want to hunt varmints, predators and deer-size game and don't want to hunt for ammo then just buy a .243. It's worked fine for 60 years, and it will work just as well in the future. If you're a reloader who takes enormous pride in tiny groups, a serious competition rifle shooter who wants every advantage possible to win matches or simply the person who enjoys have the hottest, newest cartridge then the 6mm Creedmoor is your answer.